The invention disclosed herein relates generally to protection devices used to protect other devices from damage or destruction resulting from voltage or current surges. In particular, the present invention relates to such a protection device which is implantable in the body of a patient with a heart pacemaker to protect the pacemaker against current surges, particularly those resulting from the operation of an external or implanted heart defibrillator.
It is well known that in many instances an implanted heart pacemaker can successfully regulate the otherwise faulty operation of a damaged or diseased heart. Generally, a typical pacemaker senses electrical activity or lack of such activity in the heart muscle, and supplies electrical stimulus pulses to the heart to stimulate contractions when necessary. The electrical stimulus pulses generated by a pacemaker, however, are ineffective to stop the lethal condition of fibrillation. However, it is well known that the application of a series of high-voltage pulses to the heart is often effective in arresting fibrillation. Of course it is desirable following defibrillation of the heart for the pacemaker to resume its normal regulatory role. A serious problem in this regard, however, is that without adequate protection against the large current flow induced by the application of high-voltage defibrillation pulses to the heart, a pacemaker can be damaged or destroyed. Obviously, from the standpoint of the patient's continued well being, this is a totally unacceptable consequence.
In the past, a number of attempts have been made to provide adequate protection against excessive currents and voltages for pacemakers and other medical devices such as electrocardiogram (ECG) amplifiers. For example, it is known to connect one or more zener diodes between the opposite leads of a pacemaker to limit the voltage differential therebetween.
However, as discussed in U.S. Pat. No. 4,320,763 to Money, this approach is not effective to limit the current flow between the heart tissue and the electrode at the distal end of the pacemaker lead. As a result, the heart tissue near the point of contact with the electrode can be severely damaged when high-voltage defibrillation pulses are applied to the heart. The U.S. Pat. No. 4,320,763 discloses that such tissue damage can be prevented by connecting a current limiting device such as a diode or a pair of field effect transistors (FETs) in series between a pacemaker output terminal and a distal electrode. However, it is apparent that the current limiting device thereby becomes a permanent part of the pacemaker circuit. When current limiting is not needed, for example during normal pacing operation, it is desirable to remove the current limiting device from the circuit to avoid unnecessary noise generation as well as loading effects.
An approach for protecting the pacemaker circuitry itself is disclosed in U.S. Pat. No. 4,440,172 to Langer. The U.S. Pat. No. 4,440,172 discloses an implantable pacemaker and defibrillator unit in which the pacemaker and defibrillator share common output and return lines. The pacemaker generates negative-going stimulus pulses and is protected against the positive-going high-voltage defibrillator pulses by a resistor and forward biased diode connected in series between the common output line and ground. This approach only provides limited protection to the pacemaker from unidirectional defibrillation pulses. Recent medical research has shown, however, that a number of benefits are obtained by using a bidirectional or "biphasic" pulse train to defibrillate the heart. Some of the benefits of "biphasic" defibrillation, which forms no part of the present invention, are discussed in Schuder, Defibrillation of 100 kg Calves With Asymmetrical, Bidirectional, Rectangular Pulses, Cardiovascular Research 419-426 (1984), and Jones, Decreased Defibrillator-Induced Dysfunction With Biphasic Rectangular Waveforms, Am. J. Physiol. 247 (Heart Circ. Physiol. 16): H792-H796 (1984).
U.S. Pat. No. 3,886,932 to Suessmilch discloses an approach in which a small sampling resistor is connected in series with a pacemaker lead. The resistor voltage is compared to a preselected reference voltage. If the resistor voltage exceeds the reference voltage, a switch in series with the resistor and lead is opened for a preselected period of time, after which it is closed and the voltage across the resistor again sampled. With this approach, there is no way to know what the current in the pacemaker lead will be when the switch closes at the end of the preselected time period. As a result, when the switch closes, it is possible that current flow in the lead will rise very rapidly. Even if the switch is quickly opened again, a current spike of unknown magnitude can be transmitted to the pacemaker, possibly causing damage, or causing the pacemaker sensor circuitry to misinterpret the status of the heart.
U.S. Pat. No. 4,102,348 to Hihara discloses an approach similar to that of Suessmilch except that the series switch is implemented as a bipolar transistor, the base voltage of which is supplied by the signal on the pacemaker lead through a resistive voltage divider. The state of the switch depends on the voltage of the signal on the lead. This approach, similarly to the Langer patent provides protection only against undirectional defibrillation pulses.
Accordingly, the present invention has as an object to provide a protection device that protects both a pacemaker or other implantable device and the heart tissue near a lead thereof against damage from high current and voltage levels, particularly those induced by the application of defibrillation pulses. In addition, the present invention has as an object to provide a protection device that switches into the circuit of a pacemaker or other implantable device to limit current in a lead of the device when current exceeds a safe limit, but which is effectively switched out of the circuit during normal operation of the device. Another object of the present invention is to provide a protection device that provides protection against high-voltage bidirectional or "biphasic" defibrillation pulses. Still another object of the present invention is to provide a protection device that prevents the transmission of possibly dangerous current and voltage spikes to the pacemaker or other device.